Interference mitigation for orthogonal frequency division multiplexing communication

ABSTRACT

An Orthogonal Frequency Division Multiplex (OFDM) communication system comprises OFDM transmitters, an OFDM receiver, and a subcarrier status data controller for transmitting subcarrier status data to the OFDM receiver. The subcarrier status data indicates the active subcarriers of the OFDM transmitters. The OFDM receiver comprises a receiver, a subcarrier status processor, a channel estimator, and an interference mitigation processor.

FIELD OF THE INVENTION

The invention relates to interference mitigation for orthogonalfrequency division multiplexing communication, and in particular, butnot exclusively, to interference mitigation in a cellular communicationsystem employing orthogonal frequency division multiple access.

BACKGROUND OF THE INVENTION

Wireless communication using radio frequencies has become increasinglywidespread in the last decade and many communication systems now competefor a limited resource. As a result, one of the most importantparameters for wireless communication systems is how efficiently theycan use the allocated frequency spectrum.

The requirement for an efficient use of the scarce frequency spectrumresource has led to the development of wireless technologies that canoperate with high levels of interference. For example, it is a keyrequirement for high capacity cellular communication systems that a highlevel of interference can be permitted. Typically these communicationsystems operate with a frequency reuse of one, which means that the samechannel bandwidth is available and is used in all sectors and cellsacross the network. As a result, the intercell interference seen fromthe neighbor cells can be very substantial at the cell overlap areas.Since the power available to the transmitter is constrained, theavailable Carrier to Interference Ratio (C/I) and hence the data rate isalso constrained under this condition. If the intercell interference canbe removed, the effective C/I increases and the data rate increasescommensurate with the improvement in C/I. This may provide a much higherspectral efficiency and increase the capacity of the systemsubstantially, and it is therefore highly desirable to remove ormitigate the intercell interference.

A communication scheme which may be used in wireless communicationsystems is the Orthogonal Frequency Division Multiple (OFDM) scheme.Furthermore, a cellular communication system may use OrthogonalFrequency Division Multiple Access (OFDMA) wherein users in the samecell are assigned sub-carrier groups that are simultaneously active withother user's sub-carrier groups. However, in OFDMA, transmissions withina cell may be kept orthogonal and the interference generated to users inthe same cell (intracell interference) can be effectively mitigated tothe extent that it can typically be ignored.

However, interference from other cells (intercell interference) is notorthogonal and may consequently appear as interference and degrade thetransmissions. As a consequence, it is highly desirable to mitigate theimpact of the intercell interference. Techniques for mitigatingintercell interference are well known in the art and an example of amethod of intercell interference mitigation can be found in A. E. Jonesand S. H. Wong; “Generalised Multiuser Detection in TD-CDMA”,Proceedings of IEEE Vehicular Technology Conference, Stockholm, May2005, the Institute of Electrical and Electronic Engineers, incorporatedby reference herein.

However, a problem with the known approaches for mitigation ofinterference is that the performance and efficiency is highly dependenton information relating to the interferers. This information istypically difficult to obtain for the individual receiver resulting inthe relatively inaccurate estimates or assumptions being used. As aconsequence, the interference mitigation is frequently suboptimal andresults in a significant degradation of the communication quality and areduction of the capacity of the cellular communication system.

Hence, improved interference mitigation for OFDM communication would beadvantageous and in particular a system allowing increased flexibility,improved performance, increased system capacity, facilitated operation,reduced complexity and/or improved provision of information enabling,facilitating or improving interference mitigation would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the invention seeks to preferably mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination.

According to a first aspect of the invention there is provided anOrthogonal Frequency Division Multiplex (OFDM) communication systemcomprising: a plurality of OFDM transmitters; at least one OFDM receiverfor receiving an OFDM signal from a first OFDM transmitter of theplurality of OFDM transmitters; and transmission means for transmittingsubcarrier status data to the OFDM receiver, the subcarrier status databeing indicative of active subcarriers for a number of OFDM transmittersother than the first OFDM transmitter; wherein the OFDM receivercomprises: means for receiving a signal comprising a desired signalcomponent from the first OFDM transmitter and interference from at leastone interfering OFDM transmitter of the plurality of OFDM transmitters,means for receiving the subcarrier status data, channel estimation meansfor determining channel estimates for at least an air interfacecommunication channel from the first OFDM transmitter and an airinterface communication channel from the interfering OFDM transmitter,interference mitigation means for performing mitigation of theinterference in response to the subcarrier status data and the channelestimates.

The inventors of the current invention have realized that improvedperformance can be achieved in an OFDM communication system bysignalling information indicative of which subcarriers are active for aninterfering OFDM transmitter. The invention may facilitate the provisionof information used for interference mitigation and/or may provideimproved accuracy of this information. An improved interferencemitigation may be achieved which may result in increased communicationquality and/or increased system capacity. A low complexity system and/ora practical implementation may be achieved.

The transmission means may be distributed in the communication systemand may specifically be part of the OFDM transmitters. For example, eachof the OFDM transmitters may comprise functionality for transmittingsubcarrier status data. The interference mitigating means may bearranged to regenerate the data transmitted by the first OFDMtransmitter in response to the channel estimates, the received signaland the subcarrier status data. The subcarrier status data may beprocessed before being used for interference mitigation. For example,the subcarrier status data may be decoded, divided, combined orprocessed in any suitable way.

An active subcarrier may be a carrier on which data is transmittedwhereas a non-active subcarrier may be a carrier on which no data istransmitted. The subcarrier status data may correspond to informationfor all subcarriers of an OFDM symbol or may correspond to informationfor all subcarriers or for only a subset of subcarriers.

According to an optional feature of the invention, the interfering OFDMtransmitter is arranged to transmit subcarrier status data indicative ofactive subcarriers of the interfering OFDM transmitter.

This may allow an efficient system and may allow a practicalimplementation. For example, each OFDM transmitter may transmitsubcarrier status data indicating which subcarriers it is activelyusing. Subcarrier status data for the entire OFDM symbol may bedetermined by combining subcarrier status data from a plurality of theOFDM transmitters.

According to an optional feature of the invention, at least one otherOFDM transmitter of the plurality of OFDM transmitters is arranged totransmit subcarrier status data indicative of active subcarriers of theinterfering OFDM transmitter.

This may allow an efficient system and/or may allow a practicalimplementation. Specifically it may allow subcarrier status data fore.g., a plurality of OFDM transmitters to be obtained by receiving acommunication from only a single OFDM transmitter. For example, the OFDMtransmitters may be coupled through a fixed network and at least some ofthe OFDM transmitters may comprise functionality for transmittingsubcarrier status data relating to other OFDM transmitters.

According to an optional feature of the invention, all OFDM transmittersof a group of OFDM transmitters transmit subcarrier status data for allthe OFDM transmitters of the group.

This may allow a facilitated operation of the system and/or mayfacilitate the reception of the subcarrier status data by the OFDMreceiver. In particular, the OFDM receiver may receive the subcarrierstatus data from any OFDM transmitter and is not restricted to obtainspecific subcarrier status data from a specific OFDM transmitter.

According to an optional feature of the invention, the group of OFDMtransmitters corresponds to base stations controlled by a single RadioNetwork Controller of a cellular communication system.

This may improve performance and provide low complexity, facilitatedoperation and/or simplified reception of the subcarrier status data in acellular communication system.

According to an optional feature of the invention, the group of OFDMtransmitters are arranged to transmit at least some of the subcarrierstatus data by a substantially synchronous transmission of the same OFDMsymbol.

This may improve performance and/or facilitate operation. In particular,it may allow the OFDM receiver to receive the subcarrier status datawithout requiring that it separates between signals from different OFDMtransmitters. For example, it may provide air interface combining ofsignals from different OFDM transmitters.

According to an optional feature of the invention, the means fortransmitting is arranged to repeatedly transmit subcarrier status data,each transmission of subcarrier status data relating to a time intervalin which the subcarrier status data is valid.

This may allow a flexible and efficient system where communication andmanagement overhead may be kept low.

According to an optional feature of the invention, at least theinterfering OFDM transmitter is arranged to transmit a known datasequence.

This may improve performance and/or facilitate operation orimplementation.

The known data sequence may e.g., be a pilot signal, a sequence of knownpilot symbols and/or known training data. The known data sequence may bea sequence in the time domain and/or the frequency domain. For example,the known data sequence may comprise subcarrier symbols in a pluralityof subcarriers in the same OFDM symbol as well as in consecutive OFDMsymbols.

According to an optional feature of the invention, the channelestimation means is arranged to determine the channel estimate for theinterfering OFDM transmitter in response to the known data sequence.

This may allow a particularly suitable and efficient means ofdetermining a channel estimate for the interfering OFDM transmitter.

According to an optional feature of the invention, the transmissionmeans comprises means in the interfering OFDM transmitter for selectingthe known data sequence in response to the active subcarriers for theinterfering OFDM transmitter.

This may allow a particularly efficient communication of information ofthe subcarrier status data. Specifically, the subcarrier status data maybe represented by the known data sequence selected and the requirementfor communicating explicit data for the subcarrier status data may beobviated.

According to an optional feature of the invention, the OFDM receiver isarranged to determine the subcarrier status data in response to adetection of a known data sequence of an OFDM transmission.

This may allow a particularly efficient communication of information ofthe subcarrier status data. Specifically, the subcarrier status data maybe represented by the known data sequence and the requirement forreceiving and decoding explicit data for the subcarrier status data maybe obviated. The known data sequence may be the known data sequence fromthe interfering OFDM transmitter and/or may be a known data sequencefrom a different OFDM transmitter. The subcarrier status data may e.g.,be determined by combining subcarrier status data for differentsubcarriers received from different OFDM transmitters by the known datasequence used by the individual OFDM transmitter.

According to an optional feature of the invention, disjoint sets ofknown data sequences are allocated to different OFDM transmitters.

This may facilitate determination of the origin of a received signal.

According to an optional feature of the invention, the subcarrier statusdata comprises at least one data value indicative of an activesubcarrier status for a group of subcarriers.

This may facilitate communication of the subcarrier status data and mayreduce the overhead required for the communication thereof. For example,the subcarriers may be divided into groups of larger size and a singleactive/ non-active indication may be given for the entire group. Thismay be particularly suitable for applications where subcarriers areallocated users in subcarrier groups.

According to an optional feature of the invention, the OFDMcommunication system further comprises means for transmitting anindication of a plurality of potentially interfering OFDM transmittersto the OFDM receiver.

This may facilitate operation and/or improve performance. For example,in a cellular communication system, a neighbor list may be transmittedindicating the neighbor OFDM transmitters serving adjacent cells.

According to an optional feature of the invention, the OFDM receiverfurther comprises means for evaluating received signals from a pluralityof potentially interfering transmitters, and means for selecting asubset of the plurality of potentially interfering transmitters forinterference mitigation in response to the evaluation.

This may reduce complexity, processing overhead and/or may allowimproved performance. Specifically, the feature may allow thatinterference mitigation is focused on the main interfering sources inthe specific situation and may thus allow the available interferencemitigation resource to be used most efficiently. Any suitable evaluationand selection criterion may be used. For example, the evaluation maydetermine one or more of an interference level, a receive level signal,a subcarrier interference level etc and the selection may for examplecorrespond to a selection of the N sources causing the highestinterference levels, where N is the number of interfering sources thatcan be mitigated by the interference mitigation means. The evaluationmay for example be based on transmissions of known data sequences fromthe OFDM transmitters and specifically may be based on pilot signalsfrom the OFDM transmitters.

According to an optional feature of the invention, the OFDMcommunication system further comprises means for selecting a subset ofthe subcarrier status data relating to the subset of the plurality ofpotentially interfering transmitters.

This may facilitate operation and/or reduce the computational load. Forexample, the OFDM receiver may only demodulate and decode the subcarrierstatus data corresponding to the OFDM transmitters selected for theinterference mitigation.

According to an optional feature of the invention, the OFDM receiverfurther comprises means for determining an identity of an OFDMtransmitter active on a given subcarrier in response to the subcarrierstatus data.

This may allow improved performance and may in particular improveinterference mitigation as the appropriate source for a given subcarrierinterference can be identified. For example, the subcarrier status datamay indicate that a first group of subcarriers are active for one OFDMtransmitter whereas a second group of subcarriers are active for anotherOFDM transmitter. If e.g., the first OFDM transmitter is active on boththe first and second group of subcarriers, the interference mitigationmay take the first interfering OFDM transmitter into account for thefirst group of subcarriers and the second interfering OFDM transmitterinto account for the second group of subcarriers.

According to an optional feature of the invention, the OFDM receivercomprises means for determining channel estimates for individualsubcarriers in response to the subcarrier status data.

The channel estimates may particularly be determined based on adetermination of a source of the interfering signal in a givensubcarrier in response to an identity of the OFDM transmitter(s) beingactive in this subcarrier comprised in the subcarrier status data.

This may facilitate operation and allow improved determination of theinterference scenario and thus improved interference mitigation.

According to an optional feature of the invention, the means forreceiving is arranged to receive a subset of the subcarrier status datafrom a plurality of OFDM transmitters; and the OFDM receiver comprisesmeans for determining subcarrier status data by combining the subsets ofsubcarrier status data.

This may facilitate operation and/or improve performance in manyapplications. For example, in an uplink scenario for a cellularcommunication system, the OFDM transmitters of remote units may transmitsubcarrier status data indicating which subcarriers they are allocatedand which are thus active. By combining the subcarrier status data fromthe remote units, subcarrier status data for the entire OFDM symbol maybe derived.

According to an optional feature of the invention, the mitigation meansis arranged to perform a joint determination of data symbols from atleast the first OFDM transmitter and the interfering OFDM transmitter.

This allows high performance and/or a practical implementation. Thejoint determination may be a joint detection.

According to an optional feature of the invention, the communicationsystem is a cellular communication system.

The invention may provide improved performance in a cellularcommunication system using OFDM communication techniques, and may inparticular mitigate intercell interference thereby improvingcommunication quality and/or service.

According to an optional feature of the invention, the plurality of OFDMtransmitters corresponds to a plurality of base stations.

The invention may provide improved performance in the downlink of acellular communication system and may in particular reduce the impact ofintercell interference.

According to an optional feature of the invention, the plurality of OFDMtransmitters corresponds to a plurality of remote units wherein thefirst OFDM transmitter and the interfering OFDM transmitter havedifferent serving cells.

The invention may provide improved performance in the uplink of acellular communication system and may in particular reduce the impact ofintercell interference.

According to an optional feature of the invention, the OFDM receivercomprises means for determining subcarrier status data for a neighborcell of a serving cell of the OFDM receiver.

This may facilitate and/or improve intercell interference mitigation andmay thus improve performance of the communication system as a whole.

According to an optional feature of the invention, the interferencemitigation means is arranged to determine a subcarrier activity statusfor a neighbor cell of a serving cell of the OFDM receiver from subsetsof subcarrier status data received from a plurality of OFDM transmittersof the neighbor cell.

This may facilitate and/or improve interference mitigation in a cellularcommunication system. In particular, it may facilitate the determinationof the active status of subcarriers. The feature is particularlysuitable for uplink communication.

For example, in an uplink scenario for a cellular communication system,the OFDM transmitters of remote units may transmit subcarrier statusdata indicating which subcarriers they are allocated and which are thusactive. By combining the subcarrier status data from the remote units,subcarrier status data for the entire OFDM symbol may be derived.

According to another aspect of the invention, there is provided an OFDMreceiver for receiving an OFDM signal from a first OFDM transmitter of aplurality of OFDM transmitters, the OFDM receiver comprising: means forreceiving a signal comprising a desired signal component from the firstOFDM transmitter and interference from at least one interfering OFDMtransmitter of the plurality of OFDM transmitters; means for receivingsubcarrier status data, the subcarrier status data being indicative ofactive subcarriers for a number of OFDM transmitters other than thefirst OFDM transmitter; channel estimation means for determining channelestimates for at least an air interface communication channel from thefirst OFDM transmitter and an air interface communication channel fromthe interfering OFDM transmitter; interference mitigation means forperforming mitigation of the interference in response to the subcarrierstatus data and the channel estimates.

According to another aspect of the invention, there is provided an OFDMtransmitter comprising means for transmitting subcarrier status data toa OFDM receiver, the subcarrier status data being indicative of activesubcarriers for a number of OFDM transmitters not communicating with theOFDM receiver.

According to another aspect of the invention, there is provided a methodof interference mitigation in a Orthogonal Frequency Division Multiplex(OFDM) communication system comprising a plurality of OFDM transmitters,and at least one OFDM receiver for receiving an OFDM signal from a firstOFDM transmitter of the plurality of OFDM transmitters; the methodcomprising: transmitting subcarrier status data to the OFDM receiver,the subcarrier status data being indicative of active subcarriers for anumber of OFDM transmitters other than the first OFDM transmitter; andat the OFDM receiver: receiving a signal comprising a desired signalcomponent from the first OFDM transmitter and interference from at leastone interfering OFDM transmitter of the plurality of OFDM transmitters,receiving the subcarrier status data, determining channel estimates forat least an air interface communication channel from the first OFDMtransmitter and an air interface communication channel from theinterfering OFDM transmitter, performing mitigation of the interferencein response to the subcarrier status data and the channel estimates.

These and other aspects, features and advantages of the invention willbe apparent from and elucidated with reference to the embodiment(s)described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 illustrates and example of a communication system in accordancewith some embodiments of the invention.

FIG. 2 illustrates an OFDM receiver in accordance with some embodimentsof the invention.

FIG. 3 illustrates an OFDM receiver in accordance with some embodimentsof the invention.

FIG. 4 illustrates an example of an allocation of groups of subcarriersof an OFDM symbol.

FIG. 5 illustrates an example of allocation of cell specific pilotsequences.

FIG. 6 illustrates an example of a method of interference mitigation inan Orthogonal Frequency Division Multiplex (OFDM) communication systemin accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

The following description focuses on embodiments of the inventionapplicable to a cellular communication system using OFDM techniques.However, it will be appreciated that the invention is not limited tothis application but may be applied to many other OFDM communicationsystems including, for example, OFDM based Wireless Local Area Networkssuch as IEEE 802.11.

FIG. 1 illustrates an example of a communication system in accordancewith some embodiments of the invention. FIG. 1 specifically illustratesone OFDM receiver 101 and four OFDM transmitters 103, 105, 107, 109. Itwill be appreciated that the communication system may comprise many moreOFDM receivers and transmitters and that OFDM transmitters and receiversmay be combined as OFDM transceivers. Thus, typically, mostcommunication units comprise both an OFDM receiver and an OFDMtransmitter.

In the example, the OFDM receiver 101 is receiving a desired signal froma first OFDM transmitter 103 whereas the other OFDM transmitters 105-109transmit to other OFDM receivers but in the same frequency band (forexample they may be in different cells of a cellular communicationsystem having a frequency reuse of one). Thus, the other OFDMtransmitters 105-109 create interference to the OFDM receiver 101. Inaccordance with some embodiments of the invention, this interference ismitigated thereby providing improved reception of the wanted signal fromthe first OFDM transmitter 103

An OFDM signal for the data word α=(α₁, . . . , α_(N)) can berepresented by:

$\begin{matrix}{s_{k} = {\sum\limits_{n = 1}^{N}{\lambda_{n}\alpha_{n}{\mathbb{e}}^{{- j}\frac{2{\pi{({n - 1})}}k}{T_{c}}}}}} & (1)\end{matrix}$where T_(c) is the OFDM symbol period and λ_(n)ε(0,1) defines whetherthe sub-carrier is active or inactive. The elements of the vector α aregenerally complex modulation symbols that can be modulated in both phaseand amplitude.

A subcarrier may be considered active when |α_(n)|>0 or morespecifically with reference to eq. 1 when λ_(n)=1. Similarly, asubcarrier may be considered inactive when |α_(n)|=0 or when λ_(n)=0.

Typically, the OFDM symbol is pre-pended with a cyclic prefix of lengthP. The use of cyclic prefix reduces inter-symbol interference andmaintains orthogonality in dispersive channels. The technique of cyclicprefixing OFDM symbols is well known in the art and will for brevity notbe described further. It will furthermore be appreciated that thepre-pending of a cyclic prefix is not important for the inventiveconcepts and that the following description applies equally well to OFDMcommunication not using a cyclic prefix.

The OFDM symbol with cyclic prefix is represented by the vector c=(c₁,c₂, . . . , c_(N+P)).

After transmission through the channel, and processing by the receiver,the sampled input to the OFDM demodulator is given byr=b+z  (2)where b=c{circle around (×)}h, {circle around (×)} denotes convolution,h=(h₁, h₂, . . . , h_(W)) is the channel impulse response of the airinterface communication channel between the first OFDM transmitter 103and the OFDM receiver 101, and z=(z₁, z₂, . . . , z_(N+P+W−)1) containsboth thermal noise and intercell interference from the other OFDMtransmitters 103-109.

Demodulation is achieved by using the reciprocal process to thatemployed in the transmitter via the Fast Fourier Transform (FFT). Priorto demodulation the cyclic prefix is removed (and thus the tail from thechannel dispersion is removed). The resulting signal can be representedby:r′=b′+z′  (3)

The complex time samples from an OFDM symbol are transformed by a FastFourier Transform (FFT) to produce a set of complex modulation states,one for each sub-carrier. The output of the FFT is given byu=λ×α×H+F(z)  (4)where F(.) corresponds to the FFT process and H is the frequency domainrepresentation of the channel impulse response, i.e., H=F(h). Theintercell interference plus thermal noise term F(z) can be expanded,such that:

$\begin{matrix}{{F(z)} = {{\sum\limits_{x = 1}^{X}{\gamma_{x}H_{x}}} + {F(n)}}} & (5)\end{matrix}$where for OFDM transmitter x, γ_(x)=λ_(x)×α_(x) and H_(x) are the dataword and frequency domain representation of the channel impulserespectively, and X is the number of interfering OFDM transmitters to beconsidered. The vector n=(n₁, n₂, . . . , n_(N)) contains thermal noiseand any error terms that may arise from imperfect receiver processing.

For a cellular communication system, the serving cell is denoted by thesubscript 0. By substituting eqn. 4 into eqn. 3 the output of the FFT isgiven by

$\begin{matrix}{{{u\left( {H_{0},H_{1},H_{2},\ldots\mspace{11mu},H_{X}} \right)} \times \begin{pmatrix}\lambda_{0} \\\lambda_{1} \\\lambda_{2} \\\vdots \\\lambda_{X}\end{pmatrix} \times \begin{pmatrix}\alpha_{0} \\\alpha_{1} \\\alpha_{2} \\\vdots \\\alpha_{X}\end{pmatrix}} + {F(n)}} & (6)\end{matrix}$

This output vector is in a form which is well known to be amenable formitigating intercell interference. A number of techniques will be knownto the person skilled in the art for mitigating interference based onthis representation and specifically techniques for determining α₀ byjointly detecting the data vectors α_(x).

An example of a suitable interference mitigation algorithm may forexample be found in A. E. Jones and S. H. Wong; “Generalised MultiuserDetection in TD-CDMA”, Proceedings of IEEE Vehicular TechnologyConference, Stockholm, May 2005, the Institute of Electrical andElectronic Engineers, incorporated by reference herein. This algorithmcomprises a data estimation technique based on the above representationof the FFT output.

However, the accuracy of the data estimation technique and theefficiency of the interference mitigation algorithm heavily depend onthe accuracy of the information used for the interference mitigation. Inparticular, the inventors have realized that improved performance can beachieved if improved information of the vectors λ_(x) and H_(x) can bedetermined, i.e., if improved information of the activity status of theindividual subcarriers and/or the channel estimates can be provided.

In the example of FIG. 1, the OFDM communication system comprises meansfor transmitting subcarrier status data to the OFDM receiver 101. Thesubcarrier status data is indicative of the active subcarriers for oneor more of the OFDM transmitters 105-109. Thus, the subcarrier statusdata may directly provide the activity status of one or more subcarriersof one or more of the OFDM transmitters 105-109 or may indirectlyprovide an indication allowing at least one activity status of at leastone subcarrier of at least one interfering OFDM transmitter to bedetermined.

For example, each of the OFDM transmitters 105-109 may comprise meansfor transmitting a message to the OFDM receiver 101 indicating whichsubcarriers are currently active for the specific OFDM transmitter105-109. The OFDM receiver 101 may then combine these messages togenerate subcarrier status data for all of the OFDM transmitters105-109. In this way, the subcarrier status data may be used to generateaccurate information for the vectors λ_(x) thus allowing improvedinterference mitigation.

Furthermore, the subcarrier status data may assist in the determinationof the channel estimates. For example, the subcarrier status datareceived from the different OFDM transmitters 105-109 providesinformation of which OFDM transmitter is active in which specificsubcarriers. Thus, the OFDM receiver 101 can generate the channelresponse for the interferer in a given subcarrier based on which OFDMtransmitter is currently active in that subcarrier.

Thus, by signalling the activity of the subcarriers to the OFDM receiver101 a much improved interference mitigation can be achieved resulting inimproved communication, improved carrier to effective interference ratioand an increased capacity of the cellular communication system.

In the following a more detailed description for a cellularcommunication system will be given. For clarity and brevity, thedescription will focus on an OFDM receiver 101 receiving a signal fromone OFDM transmitter 101 with one main interferer being present.However, it will be appreciated that the OFDM receiver 101 maysimultaneously receive signals from a plurality of OFDM transmitters indifferent subcarriers and that the described concepts are readilyapplied to a plurality of interferers.

FIG. 2 illustrates an OFDM receiver in accordance with some embodimentsof the invention. The OFDM receiver 101 may specifically be the OFDMreceiver 101 of FIG. 1 and will be described with reference to theexample of FIG. 1.

The invention will initially be described with reference to a downlinkscenario wherein the OFDM receiver 101 corresponds to a remote unit of acellular communication system and the OFDM transmitters 103-109correspond to base stations of the cellular communication systems. Thus,the OFDM transmitters 103-109 are in this example neighboring basestations forming neighbor cells for the OFDM receiver 101.

The remote unit may be a user equipment such as a 3^(rd) Generation UserEquipment (UE), a communication unit, a subscriber unit, a mobilestation, a communication terminal, a personal digital assistant, alaptop computer, an embedded communication processor or any physical,functional or logical communication element which is capable ofcommunicating over the air interface of the cellular communicationsystem.

In the example, the OFDM receiver 101 is in the cell served by the firstOFDM transmitter 103 and is communicating with this using a set ofsubcarrier assigned to it.

In addition, a neighbor cell base station comprises an interfering OFDMtransmitter 105 which creates intercell interference to the OFDMreceiver 101. The intercell interference may be particularly high if theOFDM receiver 101 is located close to the cell edge between the cells ofthe first OFDM transmitter 103 and the interfering OFDM transmitter 105.

In the example, the interfering OFDM transmitter 105 comprises asubcarrier status data controller 200 which is arranged to determinesubcarrier information for the base station comprising the interferingOFDM transmitter 105.

Specifically, the subcarrier status data controller 200 obtainsinformation of which subcarriers are currently allocated to remote unitsfor ongoing communications and which subcarriers are currently not inuse. In response, the subcarrier status data controller 200 generatessubcarrier status data which indicates this status. For example, thesubcarrier status data controller 200 can generate a data wordcomprising a binary value for each subcarrier indicating whether this iscurrently active or not active.

In the specific example, the interfering OFDM transmitter 105 transmitsthe subcarrier status data to the OFDM receiver 101. For example, theinterfering OFDM transmitter 105 may transmit the subcarrier status databy including the subcarrier status data binary data word in apre-allocated slot of a broadcast channel transmitted with the maximumtransmit power of the base station.

The OFDM receiver 101 comprises a receiver 201 which is operable toreceive radio signals in the desired frequency band. Thus, the receiverwill receive a signal comprising signal components from both the desiredsource of the first OFDM transmitter 103 as well as an interferingsignal component from the interfering OFDM transmitter 105. In addition,the received signal may comprise interference from other sources as wellas noise.

The receiver 201 comprises well known functionality such as filtering,amplification and time-to-frequency transforms for generating subcarrierdata symbols for the OFDM subcarriers. Thus, the receiver canspecifically comprise an FFT for generating an OFDM symbol.

The receiver 201 is furthermore operable to receive the subcarrierstatus data transmitted from the interfering OFDM transmitter 105. Forexample, in a time division system, the receiver 201 may retune to thebroadcast channel of the interfering OFDM transmitter 105 during sometime slots or frames and may decode the subcarrier status data thereon.

It will be appreciated that any suitable way of receiving the subcarrierstatus data may be used and that the OFDM receiver 101 may for examplecomprise separate receivers for receiving the OFDM data symbols and thesubcarrier status data or the subcarrier status data may be transmittedin a way that allows the subcarrier status data to be determined as partof the reception of the wanted data.

In the example of FIG. 2, the receiver 201 is furthermore coupled to asubcarrier status processor 203 and a channel estimator 205.

The subcarrier status processor 203 receives the subcarrier status datafrom the receiver 201 and may further process this data to provide asuitable format for interference mitigation. In particular, thesubcarrier status processor 203 may generate the vectors λ_(x), forexample by combining individual subcarrier status data received from thedifferent OFDM transmitters 105-109.

The channel estimator 205 determines channel estimates for at least anair interface communication channel from the first OFDM transmitter 103to the OFDM receiver 101 and for an air interface communication channelfrom the interfering OFDM transmitter 105 to the OFDM receiver 101. Thechannel estimator 205 may further generate channel estimates forcommunication channels from other interfering OFDM transmitters 107, 109which are to be considered in the interference mitigation. Thus, thechannel estimator 205 generates the channel estimate vectors H_(x).

It will be appreciated that any method of determining the channelestimates may be used without detracting from the invention. Forexample, the channel estimates may be determined in response to trainingdata transmitted from the OFDM transmitters 103-109.

The receiver 201, the subcarrier status processor 203 and the channelestimator 205 are coupled to an interference mitigation processor 207which performs interference mitigation of the received subcarriersymbols in response to the subcarrier status data and the channelestimates. Thus, the interference mitigation processor 207 is fed thereceived data as well as the subcarrier activity vectors λ_(x), and thechannel estimate vectors H_(x).

For example, the interference mitigation processor 207 may perform anestimation of the received data α₀ by jointly estimating the datavectors α_(x). as described in A. E. Jones and S. H. Wong; “GeneralisedMultiuser Detection in TD-CDMA”, Proceedings of IEEE VehicularTechnology Conference, Stockholm, May 2005, the Institute of Electricaland Electronic Engineers, incorporated by reference herein. In the abovereferenced article, the proposed interference mitigation technique isapplied to a TD-CDMA air interface, but the person skilled in the artcan readily extend this to OFDM systems.

More specifically, a method using linear processing can be appropriate,where an inverse representation of the interfering signal passed throughthe relevant channel is constructed and the received set of data samplesis processed by the inverse representation of the above therebyproducing an estimate of the data symbols with the interferencesuppressed.

FIG. 3 illustrates the OFDM receiver 101 in more detail. In the example,the receiver 201 comprises means for data pre-processing 301 whichremoves the cyclic prefix for the received OFDM symbols. After cyclicprefix removal, the received signal is fed to the FFT 303. The FFT 303removes the sub-carriers and applies the soft modulation symbols to theintercell interference mitigation algorithm in the interferencemitigation processor 207.

The output of the data pre-processing means 301 is also applied to thechannel estimator 205 which comprises a serving cell channel estimator305 and a neighbor cell channel estimator 307 which determine a channelestimate for the first OFDM transmitter 103 and the interfering OFDMtransmitter 105 respectively. It will be appreciated that although FIG.3 illustrates only a single neighbor cell channel estimator 307, aplurality of neighbor cell channel estimators would normally besupported by the channel estimator 205. The number of neighbor cellchannel estimators would typically be less than or equal to the numberof expected interferers X (such as the number of neighbor cells). Theoutputs of the channel estimators 305, 307 are channel estimates andmeasurements from the channel. The channel estimates are fed to theinterference mitigation processor 207 and the measurement information isfed to the subcarrier status processor 203.

The subcarrier status data may in some embodiments be modulated onto thesub-carriers of the received OFDM symbol, or may e.g., be multiplexedinto the OFDM transmission signal. Accordingly, the subcarrier statusprocessor 203 can comprise a data demodulator which can derive thesubcarrier status data from the received signal vector r′. In order todemodulate this signal, the demodulator of the subcarrier statusprocessor 203 is in the example of FIG. 3 coupled to the receiver 201from which it receives the output of the data pre-processing means 391as well as the output vector after FFT processing. In addition, thesubcarrier status processor 203 is coupled to the channel estimator 205from which it receives the channel measurements thereby allowing it todetermine the subcarrier status data. It will be appreciated that manydifferent techniques suitable for transmitting and receiving thesubcarrier status data are known and need not be described furtherherein. It will furthermore be appreciated that any suitable method ofcommunicating the subcarrier status data may be used without detractingfrom the invention.

For example, the subcarrier status data may be communicated by explicitor implicit signalling.

As an example of explicit signalling, the OFDM transmitters 101-109 maydirectly include the subcarrier status data in the OFDM symbols beingtransmitted. For example, the transmission of the OFDM transmitters101-109 may be divided into time intervals with each time intervalbeginning with the signalling OFDM symbol being transmitted. Thissignalling OFDM symbol can comprise the subcarrier status data for theentire time interval. E.g., within each time interval, the allocationmay be constant such that a variation in the active status of thesubcarriers can only occur at the time interval transitions. Thus, asingle OFDM symbol, for example having a binary data value in eachsubcarrier indicating if the subcarrier is active or not, may be sentfor the entire time interval resulting in a low signalling overhead.

In this way, the subcarrier status data may be repeatedly transmitted ineach time interval of the time divided communication. In each timeinterval the subcarrier status data which is transmitted relates only tothat time interval (or e.g., to a future time interval allowing the OFDMreceivers to configure themselves according to the subcarrier statusdata).

The OFDM transmitters 101-109 may be synchronized such that all OFDMreceivers are aware of when the signalling OFDM symbols are transmittedin different cells. It will be appreciated that in the example where theOFDM transmitters 101-109 are base stations, it may be difficult for theOFDM receiver 101 to receive data in transmissions from base stations ofother cells due to the lower received signal strength and relative highinterference. However, the joint data estimation of the interferencemitigation processor 207 relies on assumptions of the transmitted datafrom other cells and inherently seeks to separate the signals fromdifferent cells. Thus, the interference from the serving cell whenreceiving communication from other cells can be substantially reducedand may in principle be completely eliminated if the channel estimatesand the detected data are correct.

Furthermore, it will be appreciated that many techniques can be appliedto improve the probability of receiving the signalling OFDM symbols fromother cells. For example, as the transmission time for the OFDMsignalling symbols can be known in many embodiments, the serving cellOFDM transmitter may simply cease transmission during this timeinterval. Indeed, neighboring cells may apply a time division scheme fortransmitting the signalling OFDM symbols wherein the neighbor cells taketurns transmitting the OFDM signalling symbol while the other cellscease transmission in the meantime. Thus, effectively a time domainreuse scheme may be implemented for only the OFDM signalling symbol. Asthe OFDM signalling symbol is only transmitted for a very short durationcompared to the transmission of user data, the impact on thecommunication capacity may be negligible.

As another example, the OFDM transmitters 103-109 can increase thetransmission power When transmitting the OFDM signalling symbols or mayuse a more robust constellation order for the subcarrier data symbolvalues (such as using BPSK symbols instead of QPSK symbols).

In some embodiments, the subcarrier status data for a given cell may notbe transmitted by the base station of that cell but may alternatively oradditionally be transmitted by other base stations.

For example, the base stations comprising the OFDM transmitters 103-109are connected through a fixed network and can for example all becontrolled by the same Radio Network Controller (RNC). Thus, theinterfering OFDM transmitter 105 may communicate information of whichsubcarriers are active in the cell supported by it to the first OFDMtransmitter 103 through the fixed network. The first OFDM transmitter103 may then transmit the subcarrier status data relating to theinterfering OFDM transmitter 105 directly to the OFDM receiver 101. Thiscan provide increased communication reliability for the subcarrierstatus data of the interfering OFDM transmitter 105.

Indeed, in some embodiments, all the OFDM transmitters 103-109 belongingto a group may transmit the subcarrier status data for all the OFDMtransmitters 103-109 of the group. In the specific example, all the basestations may transmit information of the active status of thesubcarriers to their serving RNC. The RNC may then generate combinedsubcarrier status data by combining the received subcarrier status datafrom the individual base stations. The combined subcarrier status datais then fed to the base stations which subsequently transmit this overthe air interface allowing the remote units to improve the intercellinterference mitigation.

It will be appreciated that in some embodiments the subcarrierallocation may be performed in the RNC thereby obviating the need forthis to be communicated from the base stations.

In this example, all cells connected to the same RNC transmit λ_(x) forall x. Furthermore, the subcarrier status data can be transmitter by asubstantially synchronous transmission of the same OFDM signalingsymbol(s). This is equivalent to broadcasting the active sub-carriergroups for all cells in all cells that are connected to the same RNC. Anadvantage of this approach is that there is no intercell interferencefor the signalling symbols since the signal is common to all cells forthis transmission. In fact, the simultaneous transmission may improvereliability as an air interface combination of the signals fromdifferent cells occurs. For subsequent transmissions of data the networkbehaves in the usual manner in terms of intercell interference.

Thus, the communication of the subcarrier status data is not limited tothe cell to which the subcarrier status data relates but is alsotransmitted to other cells thereby facilitating intercell interferencemitigation in these cells. This transmission may for example be directlyfrom the OFDM transmitters in the other cells or may be transmitted bythe OFDM transmitter of the serving cell itself.

In some embodiments, it may be advantageous to reduce the amount ofsubcarrier status data which is communicated by grouping subcarrierstogether and only providing one activity indication for each group.

In OFDMA systems, a user is typically allocated a group of sub-carriersrather than a single sub-carrier. For example, subcarriers may only beallocated to individual remote units in blocks of a given size.Therefore, by only including one data symbol for each block ofsubcarriers a reduced size of the subcarrier status data is achieved.FIG. 4 illustrates an example of an allocation of groups of subcarriersof an OFDM symbol. In the example, a total of 28 sub-carriers areavailable. These are further divided into in 7 groups, with 4sub-carriers per group. By grouping the sub-carriers together the lengthof λ_(x) is reduced from 28 to 7 values.

In the example, an active group of sub-carriers is indicated by apattern whereas a non-active group of sub-carriers is indicated by aclear pattern. An active sub-carrier group is signalled in the bitstream by a logical 1 and an inactive sub-carrier group by a logical 0.(It will be understood that other symbols could be used for signallingthe activity of sub-carrier groups). In this example, the subcarrierstatus data may be represented by the bit stream of:λ_(x)=(1,1,0,0,0,0,1)

Many OFDM communication systems use the transmission of known datasequences to facilitate reception. For example, pilot signals are oftentransmitted by OFDM transmitters to allow receivers to determine channelestimates with improved accuracy and less complexity. Such known datasequences may in the example of FIGS. 1 to 3 be used to determine thechannel estimates.

In some embodiments, the subcarrier status data may furthermore beimplicitly communicated by the known data sequences. Specifically, anOFDM transmitter may select a known data sequence from a set of knowndata sequences depending on the active status of the subcarriers. As aspecific example corresponding to the example of FIG. 4, a set of 128known data sequences may be allocated to the base station transmittingthe signal shown in FIG. 4. The determined λ_(x)=(1,1,0,0,0,0,1) maythen be used to access a look-up table comprising the 128 known datasequences and the corresponding data sequence may be selected. Thus, theselected known data sequence represents the subcarrier status data.

The receiver may correlate the received input signal with all possibleknown data sequences and may select the sequence indicating the highestcorrelation. This detection may then directly be used to determine thesubcarrier status data, for example by a look-up in a locally storedassociation between known data sequences and subcarrier status data.

As a specific example, the cellular communication system of the figuresmay support a number of different pilot sequences. These may be dividedinto a number M of pilot sequence sets, each containing a number m ofpilot sequences. The pilot sequence sets can be allocated to each basestation in a suitable reuse pattern. Thus, the system may effectivelysupport cell specific pilot sequences (or sets of pilot sequences).

FIG. 5 illustrates an example of allocation of cell specific pilotsequences. In this example, 128 unique pilot sequences are segmentedinto 32 sets of 4 pilot sequences, so M=32 and m=4. An association isdefined between a cell address and the pilot set index, this isillustrated in FIG. 5, i.e., CELL ADDRESS 2 corresponds to SET INDEX 2.

In the cellular communication system, each cell is assigned a celladdress and hence at least one of the M sets of pilot sequences.Typically in a cellular communication system, the cell addresses followa reuse pattern and hence the M sets of pilot sequences also follow areuse pattern. Each pilot sequence within the assigned set may beassigned to an antenna for the downlink case, and for the uplink caseeach pilot sequence may be assigned to each user. The assigning ofpilots could be flexible and the defined mappings can be knownthroughout the network

The pilot sequences may be in the frequency domain and/or in the timedomain depending on the specific embodiment.

In such a system, if an OFDM receiver is aware of the cell addressesbeing used in the local vicinity (for example by receiving a neighborlist containing cell addresses) then it has sufficient information toderive the pilot sequence sets being used on those cells.

The subcarrier status data may be implicitly signalled by the selectionof the specific pilot sequence which has been assigned to a givensubcarrier status data vector. Thus, each of the OFDM transmitters103-109 selects the pilot sequence of the set allocated to themdepending on the current subcarrier activity. As the sets are disjoint,the pilot sequences will inherently be different.

The receiver may use these different pilot sequences to determine thechannel estimates for each OFDM transmitter 103-109. Specifically theOFDM receiver 101 may correlate the received signal with all possiblepilot sequences. It may then proceed to evaluate each set of pilotsequences and select the highest correlation in each. This pilotsequence may then be used to determine the channel estimate for the OFDMtransmitter allocated that set, and may also be used to directlyindicate the subcarrier status data for that OFDM transmitter. Thus atransmission of the subcarrier status data without any additionaloverhead can be achieved (assuming that a sufficient number of pilotsequences are available).

In the following, a specific example of the operation of the OFDMreceiver 101 will be described.

The serving base station will in most cellular communication systemstransmit a neighbor list of neighbor cells. The OFDM receiver 101 mayreceive this neighbor list and may use this list as an indication ofwhich cells to consider for interference mitigation, i.e., as anindication of a plurality of potentially interfering OFDM transmitters.In the example, there are X neighbor cells on the list. Each neighborcell is given an address and this address is associated with one of theM sets of pilot sequences. The association between neighbor celladdresses and sets of pilot sequences is predefined as indicated in FIG.5.

Accordingly, the OFDM receiver 101 is aware of the pilot sequence setsbeing used by the neighbor cells and using this information, the OFDMreceiver 101 is able to monitor the serving cell and the neighbor cellsdefined on the list. The process of monitoring can consist of estimatingthe channel impulse response for the pilot sequences of interest (bothserving cell and neighbor cells) and deriving metrics from the estimatedchannel impulse responses, e.g., power and intercell interference. Thechannel estimation may specifically comprise a correlation of thereceived signal with a local replica of the pilot sequence(s).

Using the determined metrics, the OFDM receiver 101 then selects theneighbor cells which are to be included in the interference mitigation.Typically a comparison of power indications may be used, such as forexample a selection of the OFDM transmitters which result in the highestcorrelation with the corresponding pilot sequences. However, it will beappreciated that any evaluation and selection criterion may be usedwithout detracting from the invention.

As a specific example, the interference mitigation processor 207 may becapable of simultaneously estimating data from N sources and the N−1neighbor cells which result in the highest correlation value may beselected for the interference mitigation.

The OFDM receiver 101 may then proceed to determine the subcarrieractivity vectors λ_(x) for the selected subset of OFDM transmitters. Asthe pilot sequence for each neighbor cell has already been identified,the vector may simply be obtained by retrieving the predefined datacorresponding to the pilot sequence. The channel estimates for the firstOFDM transmitter 103 and the selected interfering OFDM transmitters105-109 are then fed to the interference mitigation processor 207together with the corresponding subcarrier status data. The interferencemitigation processor 207 then proceeds to perform the interferencemitigation thereby generating the data received from the first OFDMtransmitter 103.

The previous description has focused on a downlink application in acellular communication system. However, it will be appreciated that thedescribed concepts are equally applicable to an uplink scenario. Thus,the concepts are equally applicable to a scenario wherein the OFDMreceiver 101 is a base station and the OFDM transmitters 103-109 of FIG.1 are remote units of a cellular communication system. In this example,at least the first OFDM transmitter and the interfering OFDM transmitter105 have different serving cells.

In such an embodiment, the OFDM receiver 101 may generate subcarrierstatus data for a neighbor cell by combining subcarrier status datareceived from different sources. For example, the OFDM transmitters105-109 may all be in the same neighbor cell of the first OFDMtransmitter 103. Each of the OFDM transmitters 105-109 may transmitsubcarrier status data relating to the subcarrier status of theirrespective transmissions. For example, each of the OFDM transmitters105-109 may transmit a binary data word corresponding to the OFDMsymbols with a binary data value of 1 if the corresponding subcarrier isactive for that OFDM transmitter and a binary data value of 0 if thecorresponding subcarrier is not active.

The OFDM receiver 101 may thus receive the data words from the differentOFDM transmitters 105-109 and may perform a digital OR function todetermine a subcarrier status data vector indicative of whichsubcarriers are active in the neighbor cell.

Furthermore, the received subcarrier status data from the different OFDMtransmitters 105-109 may be used to determine which channel estimatesare used for the individual subcarriers.

Thus, for the subcarriers that the OFDM transmitter 105 has indicatedare active, the channel estimate for that OFDM transmitter 105 is used.Similarly, for the subcarriers which the OFDM transmitter 107 hasindicated are active, the channel estimate for that OFDM transmitter 107is used etc. Thus, the appropriate channel estimates for the individualsubcarriers may be provided to the interference mitigation processor 207thereby resulting in improved interference mitigation and an efficientand low complexity system.

In the example, the identity of an interfering OFDM transmitter may bedetermined for the individual subcarrier (or group of subcarriers) inresponse to the received subcarrier status data.

FIG. 6 illustrates an example of a method of interference mitigation inan Orthogonal Frequency Division Multiplex (OFDM) communication systemin accordance with some embodiments of the invention. The method isapplicable to the system of FIG. 1 where the OFDM receiver 101 isreceiving data from the first OFDM transmitter 103 and will be describedwith reference to this example.

The method initiates in step 601 wherein subcarrier status data istransmitted to the OFDM receiver 101. The subcarrier status data may forexample be transmitted from the OFDM transmitters 103-109. Thesubcarrier status data is indicative of active subcarriers for one ormore of the OFDM transmitters 105-109.

Step 601 is followed by step 603 wherein the receiver 201 receives asignal comprising a desired signal component from the first OFDMtransmitter 103 and interference from at least one interfering OFDMtransmitter 105.

Step 603 is followed by step 605 wherein the receiver 201 and thesubcarrier status processor 203 receive the subcarrier status data.

Step 605 is followed by step 607 wherein the channel estimator 205determines channel estimates for at least an air interface communicationchannel from the first OFDM transmitter 103 and an air interfacecommunication channel from the interfering OFDM transmitter 105.

Step 607 is followed by step 609 wherein the interference mitigationprocessor 207 performs mitigation of the interference in response to thesubcarrier status data and the channel estimates thereby generating thedata transmitted from the first OFDM transmitter 103.

It will be appreciated that the above description for clarity hasdescribed embodiments of the invention with reference to differentfunctional units and processors. However, it will be apparent that anysuitable distribution of functionality between different functionalunits or processors may be used without detracting from the invention.For example, functionality illustrated to be performed by separateprocessors or controllers may be performed by the same processor orcontrollers. Hence, references to specific functional units are only tobe seen as references to suitable means for providing the describedfunctionality rather than indicative of a strict logical or physicalstructure or organization.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. The inventionmay optionally be implemented at least partly as computer softwarerunning on one or more data processors and/or digital signal processors.The elements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units and processors.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention:In the claims, the term comprising does not exclude the presence ofother elements or steps.

Furthermore, although individually listed, a plurality of means,elements or method steps may be implemented by e.g., a single unit orprocessor. Additionally, although individual features may be included indifferent claims, these may possibly be advantageously combined, and theinclusion in different claims does not imply that a combination offeatures is not feasible and/or advantageous. Also the inclusion of afeature in one category of claims does not imply a limitation to thiscategory but rather indicates that the feature is equally applicable toother claim categories as appropriate. Furthermore, the order offeatures in the claims does not imply any specific order in which thefeatures must be worked and in particular the order of individual stepsin a method claim does not imply that the steps must be performed inthis order. Rather, the steps may be performed in any suitable order. Inaddition, singular references do not exclude a plurality. Thusreferences to “a”, “an first”, “second”, etc. do not preclude aplurality.

1. An Orthogonal Frequency Division Multiplex (OFDM) communicationsystem comprising: a plurality of OFDM transmitters; at least one OFDMreceiver for receiving an OFDM signal from a first OFDM transmitter ofthe plurality of OFDM transmitters; and at least one transmitter fortransmitting a subcarrier status data to the OFDM receiver, wherein thesubcarrier status data is indicative of active subcarriers for a numberof OFDM transmitters other than the first OFDM transmitter; wherein theOFDM receiver comprises: a receiver for receiving a signal comprising adesired signal component from the first OFDM transmitter andinterference from at least one interfering OFDM transmitter of theplurality of OFDM transmitters, and for receiving the subcarrier statusdata, channel estimator for determining channel estimates for at leastan air interface communication channel from the first OFDM transmitterand an air interface communication channel from the at least oneinterfering OFDM transmitter, a processor performing mitigation of theinterference using the subcarrier status data for the number of OFDMtransmitters other than the first OFDM transmitter and the channelestimates for the at least one interfering OFDM transmitter.
 2. The OFDMcommunication system of claim 1 wherein the at least one interferingOFDM transmitter is configured to transmit subcarrier status dataindicative of active subcarriers of the at least one interfering OFDMtransmitter.
 3. The OFDM communication system of claim 1 wherein atleast one other OFDM transmitter of the plurality of OFDM transmittersis configured to transmit subcarrier status data indicative of activesubcarriers of the at least one interfering OFDM transmitter.
 4. TheOFDM communication system of claim 1 wherein all OFDM transmitters of agroup of OFDM transmitters transmit subcarrier status data for all theOFDM transmitters of the group.
 5. The OFDM communication system ofclaim 4 wherein the group of OFDM transmitters corresponds to basestations controlled by a single Radio Network Controller of a cellularcommunication system.
 6. The OFDM communication system of claim 4wherein the group of OFDM transmitters are configured to transmit atleast some of the subcarrier status data by a substantially synchronoustransmission of the same OFDM symbol.
 7. The OFDM communication systemof claim 1 wherein the at least one transmitter is configured torepeatedly transmit subcarrier status data, each transmission ofsubcarrier status data relating to a time interval in which thesubcarrier status data is valid.
 8. The OFDM communication system ofclaim 1 wherein the at least one interfering OFDM transmitter isconfigured to transmit a known data sequence.
 9. The OFDM communicationsystem of claim 8 wherein the channel estimator is configured todetermine the channel estimate for the at least one interfering OFDMtransmitter in response to the known data sequence.
 10. The OFDMcommunication system of claim 8 wherein the at least one transmitter isarranged in the at least one interfering OFDM transmitter to select theknown data sequence in response to the active subcarriers for the atleast one interfering OFDM transmitter.
 11. The OFDM communicationsystem of claim 1 wherein the OFDM receiver is configured to determinethe subcarrier status data in response to a detection of a known datasequence of an OFDM transmission.
 12. The OFDM communication system ofclaim 1 wherein disjoint sets of known data sequences are allocated todifferent OFDM transmitters.
 13. The OFDM communication system of claim1 wherein the subcarrier status data comprises at least one data valueindicative of an active subcarrier status for a group of subcarriers.14. The OFDM communication system of claim 1 further comprising atransmitter for transmitting an indication of a plurality of potentiallyinterfering OFDM transmitters to the OFDM receiver.
 15. The OFDMcommunication system of claim 1 wherein the OFDM receiver furthercomprises a processor for evaluating received signals from a pluralityof potentially interfering transmitters, and for selecting a subset ofthe plurality of potentially interfering transmitters for interferencemitigation in response to the evaluation.
 16. The OFDM communicationsystem of claim 15 wherein the processor selects a subset of thesubcarrier status data relating to the subset of the plurality ofpotentially interfering transmitters.
 17. The OFDM communication systemof claim 1 wherein the OFDM receiver is arranged to determine anidentity of an OFDM transmitter active on a given subcarrier in responseto the subcarrier status data.
 18. The OFDM communication system ofclaim 1 wherein the OFDM receiver is arranged to determine channelestimates for individual subcarriers in response to the subcarrierstatus data.
 19. The OFDM communication system of claim 1 wherein thereceiver is configured to receive a subset of the subcarrier status datafrom a plurality of OFDM transmitters; and the OFDM receiver is arrangedto determine subcarrier status data by combining the subsets ofsubcarrier status data.
 20. The OFDM communication system of claim 1wherein the processor is configured to perform a joint determination ofdata symbols from at least the first OFDM transmitter and theinterfering OFDM transmitter.
 21. The OFDM communication system of claim1 wherein the communication system is a cellular communication system.22. The OFDM communication system of claim 21 wherein the plurality ofOFDM transmitters correspond to a plurality of base stations.
 23. TheOFDM communication system of claim 21 wherein the OFDM receiver isarranged to determine subcarrier status data for a neighbor cell of aserving cell of the OFDM receiver.
 24. The OFDM communication system ofclaim 21 wherein the processor is configured to determine a subcarrieractivity status for a neighbor cell of a serving cell of the OFDMreceiver from subsets of subcarrier status data received from aplurality of OFDM transmitters of the neighbor cell.
 25. The OFDMcommunication system of claim 21 wherein the plurality of OFDMtransmitters correspond to a plurality of remote units wherein the firstOFDM transmitter and the interfering OFDM transmitter have differentserving cells.
 26. An OFDM receiver for receiving an OFDM signal from afirst OFDM transmitter of a plurality of OFDM transmitters, the OFDMreceiver comprising: a receiver for receiving a signal comprising adesired signal component from the first OFDM transmitter andinterference from at least one interfering OFDM transmitter of theplurality of OFDM transmitters; and for receiving subcarrier status datafrom a transmitter, wherein the subcarrier status data is indicative ofactive subcarriers for a number of OFDM transmitters other than thefirst OFDM transmitter; channel estimator for determining channelestimates for at least an air interface communication channel from thefirst OFDM transmitter and an air interface communication channel fromthe at least one interfering OFDM transmitter; processor for performingmitigation of the interference using the subcarrier status data for thenumber of OFDM transmitters other than the first OFDM transmitter andthe channel estimates for the at least one interfering OFDM transmitter.27. The OFDM receiver of claim 26 wherein the channel estimator isconfigured to determine the channel estimate for the at least oneinterfering OFDM transmitter in response to a known data sequence. 28.The OFDM receiver of claim 26 wherein the OFDM receiver is arranged todetermine the subcarrier status data in response to a detection of aknown data sequence of an OFDM transmission.
 29. The OFDM receiver ofclaim 26 wherein the OFDM receiver is arranged to evaluate receivedsignals from a plurality of potentially interfering transmitters, andselect a subset of the plurality of potentially interfering transmittersfor interference mitigation in response to the evaluation.
 30. The OFDMreceiver of claim 29 further arranged to select a subset of thesubcarrier status data relating to the subset of the plurality ofpotentially interfering transmitters.
 31. The OFDM receiver of claim 26wherein the OFDM receiver is arranged to determine an identity of anOFDM transmitter active on a given subcarrier in response to thesubcarrier status data.
 32. The OFDM receiver of claim 26 wherein theOFDM receiver is arranged to determine channel estimates for individualsubcarriers in response to the subcarrier status data.
 33. The OFDMreceiver of claim 26 arranged to receive a subset of the subcarrierstatus data from a plurality of OFDM transmitters; and the OFDM receiveris arranged to determine subcarrier status data by combining the subsetsof subcarrier status data.
 34. The OFDM receiver of claim 26 wherein theprocessor is arranged to perform a joint determination of data symbolsfrom at least the first OFDM transmitter and the interfering OFDMtransmitter.
 35. The OFDM receiver of claim 26 wherein the OFDM receiveris arranged to determine subcarrier status data for a neighbor cell of aserving cell of the OFDM receiver.
 36. A method of interferencemitigation in a Orthogonal Frequency Division Multiplex (OFDM)communication system comprising a plurality of OFDM transmitters, and atleast one OFDM receiver for receiving an OFDM signal from a first OFDMtransmitter of the plurality of OFDM transmitters, the methodcomprising: transmitting subcarrier status data to the at least one OFDMreceiver, wherein the subcarrier status data is indicative of activesubcarriers for a number of OFDM transmitters other than the first OFDMtransmitter; and at the OFDM receiver: receiving a signal comprising adesired signal component from the first OFDM transmitter andinterference from at least one interfering OFDM transmitter of theplurality of OFDM transmitters, receiving the subcarrier status data,determining channel estimates for at least an air interfacecommunication channel from the first OFDM transmitter and an airinterface communication channel from the at least one interfering OFDMtransmitter, performing mitigation of the interference using thesubcarrier status data for the number of OFDM transmitters other thanthe first OFDM transmitter and the channel estimates for the at leastone interfering OFDM transmitter.
 37. The method of claim 36 comprisingthe at least one interfering OFDM transmitter transmitting subcarrierstatus data indicative of active subcarriers of the at least oneinterfering OFDM transmitter.
 38. The method of claim 36 wherein atleast one other OFDM transmitter of the plurality of OFDM transmitterstransmits subcarrier status data indicative of active subcarriers of theat least one interfering OFDM transmitter.
 39. The method of claim 38wherein the OFDM receiver determines the subcarrier status data inresponse to a detection of a known data sequence of an OFDMtransmission.
 40. The method of claim 36 wherein all OFDM transmittersof a group of OFDM transmitters transmit subcarrier status data for allthe OFDM transmitters of the group.
 41. The method of claim 36 whereinthe at least one interfering OFDM transmitter transmits a known datasequence.
 42. The method of claim 41 wherein the channel estimate forthe interfering OFDM transmitter is determined in response to the knowndata sequence.
 43. The method of claim 41 wherein the at least oneinterfering OFDM transmitter selects the known data sequence in responseto the active subcarriers for the at least one interfering OFDMtransmitter.
 44. The method of claim 36 wherein the subcarrier statusdata comprises at least one data value indicative of an activesubcarrier status for a group of subcarriers.
 45. The method of claim 36comprising transmitting an indication of a plurality of potentiallyinterfering OFDM transmitters to the OFDM receiver.
 46. The method ofclaim 36 wherein, at the OFDM receiver, wherein the method furthercomprises evaluating received signals from a plurality of potentiallyinterfering transmitters, and selecting a subset of the plurality ofpotentially interfering transmitters for interference mitigation inresponse to the evaluation.
 47. The method of claim 46 comprisingselecting a subset of the subcarrier status data relating to the subsetof the plurality of potentially interfering transmitters.
 48. The methodof claim 36 comprising the OFDM receiver further determining an identityof an OFDM transmitter active on a given subcarrier in response to thesubcarrier status data.
 49. The method of claim 36 comprising the OFDMreceiver determining channel estimates for individual subcarriers inresponse to the subcarrier status data.